This invention relates to apparatus for use in a borehole traversing subsurface earth formations, and more particularly to apparatus for acquiring signals at different levels within a large diameter borehole.
The Vertical Seismic Profile (VSP) technique uses a combination of geophysical seismic and well logging techniques to obtain data. In addition to measuring the elapsed time for a surface-activated energy source pulse to travel to a subsurface geophone, the VSP technique records data beyond the first arrival of the propagating pulse and records all subsequent pulses as well. Because the VSP geophone is placed downhole rather than on the earth's surface, the VSP data images formation properties with greater detail than conventional seismic data.
In conducting a VSP operation, data must be collected by "anchoring" a geophone at a plurality of depth locations within a borehole. Once the instrument is anchored in position within a borehole tension is released from the cable and a seismic impulse is generated at or near the earth's surface. Related signals are detected at the geophone.
One apparatus for conducting the VSP operation is described in U.S. Pat. Nos. 4,563,757 and 4,575,831. This apparatus utilizes a sonde suspended in the borehole from a cable and having an anchoring pad. The pad is anchored to one side of the borehole wall to force the sonde into contact with the formation. While such an apparatus can be utilized in normal diameter boreholes, it is unsuitable for use in relatively large diameter boreholes, those in excess of approximately one (1) meter. In the large borehole environment an instrument of this design is not capable of exerting sufficient force on the pad member to prevent slippage of the sonde along the borehole wall when tension is removed from the cable.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing apparatus for rigidly anchoring a detector at locations within a large diameter borehole.